Thyroid Gland

The thyroid gland, the largest of the endocrine glands, is located in the neck just below the thyroid cartilage of the larynx. It consists of two lobes, one on either side of the trachea, joined by a narrow band or isthmus. It is composed of numerous hollow ball-shaped follicles with small, interspersed clusters of parafollicular cells.

Follicle cells concentrate and attach iodine to the amino acid tyrosine, producing two forms of thyroid hormone (TH): thyroxine or tetraiodothy- ronine (T₄), and smaller amounts of triiodothyronine (T₃). Manufacture and release of these hormones into the blood is regulated by thyroid stimulat­ing hormone, TSH, from the pituitary gland. The major effect of TH is to stimulate activity of enzymes involved in energy production through the oxidation (burning) of glucose, thus increasing basal metabolic rate. A side effect of this increased activity is the production of body heat.

Overactivity of the thyroid gland, called hyperthyroidism, causes ele­vated metabolic rate, nervousness, and weight loss. The most common form of hyperthyroidism, Graves’ disease, is an autoimmune disease; it is ac­companied by swelling of the thyroid (goiter) and bulging of the eyes (ex­ophthalmos). Adult hypothyroidism, or underactive thyroid, causes myxedema, characterized by lowered metabolic rate, sluggishness, and weight gain. Additional consequences of low TH levels in infants are stunted growth and irreversible brain damage. Hypothyroidism resulting from low iodine intake, with consequent low TH manufacture, produces an enlargement of the thyroid gland called endemic goiter. TSH is responsible for this enlargement.­

A color-enchanced scintigram (gamma scan) of a human thyroid gland.

Parafollicular cells produce the hormone calcitonin, which lowers blood calcium levels by suppressing the activity of bone-destroying cells called os­teoclasts, and stimulating calcium uptake by bones. Calcitonin is important in children, where growing bones are being constantly remodeled. It has lit­tle effect on the normal adult skeleton, but may be prescribed in nasal spray form to help reduce bone destruction in osteoporosis. Parathormone, pro­duced by the parathyroid gland, has opposing effects on blood calcium lev­els.

References

Marieb, Elaine Nicpon. Human Anatomy and Physiology, 5th ed. Boston: Addison-Wes- ley-Longman, 2001.

Saladin, Kenneth S. Anatomy and Physiology: The Unity of Form and Function, 2nd ed. Boston: McGraw-Hill, 2001.